Ants on a Stick

Six ants walk at 1 cm/s along a very narrow stick 100 cm long, which spans positions 0 to 100 cm. They start at the following positions:

• Ant 1: position 0, facing right
• Ant 2: position 20, facing right
• Ant 3: position 30, facing left
• Ant 4: position 40, facing right
• Ant 5: position 60, facing right
• Ant 6: position 80, facing left

When two ants meet, they instantly turn around and walk in the opposite direction at the same speed. An ant leaves the stick when it reaches either end (position 0 or 100). How many seconds does it take until the last ant has walked off the stick?

Give the answer as a number of seconds.